Do the Benefits Outweigh Mercury’s Dangers?

Indulging in fresh, elegantly prepared tuna sushi after a long day feels both pampering and healthy. Virtuous, even. But new findings about elevated levels of mercury, especially in those who consume lots of fish, may make you want to lower your fork…

Most of us have heard the back and forth about fish for years: It’s great for us, a heart-healthy food that most of us should eat more of. Or… it’s bad for us, full of mercury that may harm our brains. The various updates can be confusing, like much health news often is. So we simply eat whatever we want, albeit scratching our heads. But when it comes to fish, the contradictions are true: Too much fish or too little fish, and our health may suffer.

This conundrum has been brought back into focus by the latest findings from New York City’s Health and Nutrition Examination Survey by the New York Department of Health, the first such survey conducted by a U.S. city. Based on answers from 1,811 adults, the 2004 survey found that one in four adults had elevated blood mercury levels.

Who’s Affected

In general, the levels detected pose little risk to adults. But elevated blood mercury levels in expectant mothers can lead to cognitive delays in their children. In the survey, women ages 20-29 – a common childbearing age – had three times the national average of 0.83 micrograms of mercury per liter. (See related article: Mercury Rising: Seafood Safety Tips)

Blood mercury levels were highest among the affluent population and Asian females, particularly Chinese women. And the levels were higher in New York City than nationally, despite similar levels of consumption. Researchers speculate that either New Yorkers are eating fish with higher levels of mercury, or fish isn’t the only thing in New York spreading mercury around.

Still, mercury levels do tend to correlate with fish consumption. The more fish people ate, the higher the levels. Those who ate three or fewer fish meals during the week averaged blood levels below what’s considered safe, while those eating more than three fish meals per week averaged levels above the threshold.

What’s so Bad About Mercury?

What difference does it make if your mercury levels are high? The answer requires a little geology and biology.

Mercury is a naturally occurring element that enters the environment either organically or from human sources such as coal burning or chlorine production. Oceans contain millions of tons of naturally occurring mercury, much of it released by erosion of rocks, volcanic eruptions and deep-sea vents. Additionally, atmospheric mercury (either evaporated or released in the air through industrial processes or forest fires) can re-enter water systems through rain and snow.

A small percentage of mercury that reaches the oceans and lakes, naturally or not, is converted to methlymercury, an organic mercury compound. Fish feed on smaller organisms that have accumulated methylmercury in their systems. As the larger, predatory fish eat more of these creatures (as well as small fish), they add to the methylmercury in their bodies. And then, in turn, we consume the fish.

Methylmercury can cross the blood-brain barrier, a protective barrier formed by the blood vessels that keeps some substances from entering the brain tissue. When mercury crosses into the brain, it can interfere with the way the central nervous system works, particularly in fetuses and children, causing deficits in language, attention and memory among children.

Methylmercury in adults can affect vision, motor function and memory, as well as increase the risk of cardiovascular disease. Unfortunately, once we’ve consumed the substance, it hangs around indefinitely.

Is there evidence that mercury levels in fish are rising? A 2003 Princeton University study found that mercury levels in recently caught tuna were similar to the levels found in tuna captured in the 1970s. And not all studies predict dire effects. For instance, a 12-year study of people living on the Seychelles Islands off the east coast of Africa found no negative effects from methylmercury exposure, despite the islanders’ fish-rich diet. And the mercury concentrations found in the islanders’ hair strands – another way to measure its accumulation – were 10 times those of the U.S. population.

The U.S. Department of Energy claims that mercury emissions are also coming down, primarily because manufacturers have reduced the use of mercury in batteries, fungicides and paints. The reduction may also be the result of tighter regulations on emissions.

What’s so Good About Fish?

If fish contain mercury, and mercury gets at our brains and our children’s brains, why are we still eating it? The answer is that fish is plenty good for us. It’s a low-calorie source of protein and omega-3 fatty acids, which are essential to the growth of the fetal human brain.

Omega-3 fatty acids are really the stars of a fish dinner. Of three types – ALA(alpha-linolenic acid), DHA (docosahexaenoic acid) and EPA ((eicosapentaenoic acid) – fish have the two most important ones, EPA and DHA. ALA, found in flaxseeds, soybeans, walnuts, and other foods, converts to EPA and DHA, which are more easily used by the body. But the only direct way to get EPA and DHA is from oily cold-water fish like salmon, mackerel and halibut.

Researchers have found that DHA and EPA offer special benefits, particularly in terms of heart health. They reduce the likelihood of stroke, raise good cholesterol and lower the bad, as well as lower triglycerides, the fat in your blood. They may also reduce blood pressure and have therapeutic effects on other conditions, including inflammatory ones such as arthritis.

On its own, DHA is an important component of our brain’s cell membranes and is transferred from mother to fetus at a high rate during the last trimester when the brain is growing rapidly. EPA, in turn, is also essential for fetal and child brain growth and nervous system development.

Therefore, overreacting to calls for cutting back on fish has its own dangers. In fact, in early 2001 advocacy groups issued a number of fish advisories about the risk to pregnant women, and consumption dropped dramatically, not a good thing for expectant mothers whose fetuses need omega-3 fatty acids for proper nervous system growth and maturation.

What’s the Solution?

For maximum heart health benefits, people should eat six ounces of fatty fish at least two times a week and follow these six guidelines:

1. Choose fish low in mercury (see next page)

2. Don’t eat fish high in mercury (see next page)

3. Eat fewer, or smaller, servings of fish

4. Choose smaller-sized fish

5. Eat a variety of fish

6. Pregnant or breastfeeding women and children under six should avoid any fish that is high in mercury such as mackerel, shark or swordfish. These women should also limit their fish to no more than two meals per week of low-mercury fish, about 12 ounces total.

Speaking of keeping your food safe to eat, how well do your food safety skills stack up? Food poisoning, whether contracted in a restaurant or in the comfort of your own home, is never fun. But are you unnecessarily putting yourself at risk for food-borne illness? Take our food safety quiz and find out.

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